1. Field of the Invention
This invention relates generally to a method and system for testing oil well cement samples, and, more specifically, to estimating the compressive strength of a foam cement sample.
2. Background
It is important to know the properties of a cement formulation to ensure its acceptability for a given application, and to be meaningful, tests must simulate actual job conditions. For numerous reasons, the compressive strength of cement used in oil field applications must be known.
Historically, in order to determine the compressive strength of cement as a function of time a multiplicity of samples of the cement have been prepared in small test cylinders, or cubes. The samples are subsequently crushed as a function of time as the cement cures. In testing batches of cement in this manner, while the cement may be cured at elevated temperatures and pressures such as are present in a wellborn environment, it is necessary to remove the cement samples from the heated pressure vessel in order to perform the crush test on the strength measuring machines required for this purpose. Thus the actual testing of cement samples is performed usually at room temperature and at atmospheric pressure. Such mechanical testing is inconvenient, cumbersome and time consuming and may in some circumstances be of questionable accuracy.
While crush tests on hardened cement samples may be used to mechanically determine the compressive strength of a cement formulation, a more recent method involves subjecting a cement or cement slurry sample to simulated oil field temperatures and pressures, measuring the transit time of an acoustic signal transmitted through the sample, and correlating the transit time to compressive strength using empirically developed equations. Compressive strength measurements using this method are often taken in what is known as the Ultrasonic Cement Analyzer (UCA). The UCA generally consists of a high temperature-high pressure autoclave, a heat jacket capable of heating rates up to 5.6xc2x0 C. (10xc2x0 F.) per minute, a pair of ultrasonic transducers (preferably but not necessarily operating at a frequency of about 400 kHz) for measuring the transit time of an acoustic signal transmitted through the slurry, plus associated hydraulic plumbing. The two transducers make transit time measurements through the cement as it sets. A short pulse on a lower transducer propagates through the cement to an upper transducer. Set time and compressive strength are calculated from measured transit time via empirically developed equations. U.S. Pat. Nos. 4,259,868 and 4,567,765 disclose the UCA in detail and are incorporated herein by reference.
Current practices in oilfield cementing technology provide for the introduction of various amounts of gas in cement to produce foam cement. Foam cements have been developed for oilfield applications to solve problems associated with weak formations and fluid flow problems, particularly in deep water applications. Presently, in areas where weak zones will support only a limited height of normal-density cement without breaking down, stage tools or other techniques are used to obtain the required cement sheath without producing a hydrostatic pressure that will fracture the formation. These techniques require multiple applications of cement, which is very expensive. The development of foam cements allows production of cements having densities as low as 8.0 ppg rather than the typical 12.5 ppg of a light weight non-foam cement.
It remains important, though, to know the compressive strength of a foam cement. Unfortunately, excessive attenuation of the acoustic signal through the foam cement sample often renders the typical measurement method useless. Moreover, even if an adequate signal were available for measurement, generating and maintaining a foam cement in a vessel at high pressure is problematic and requires expensive equipment.
It may thus be appreciated that there exists a need for a manner of nondestructively testing a foam cement or foam cement slurry sample to determine the compressive strength thereof.
The present inventors have discovered that foam cement compressive strength is related to the base cement compressive strength and the volume percent foam. Applying this discovery, the present invention provides a method of estimating the compressive strength of a foam cement sample from a parametric measurement (base cement compressive strength) obtainable from conventional equipment, wherein standard laboratory measurements are used to establish the relationship between the base cement compressive strength, the volume percent entrained gas and the compressive strength of the subject foam cement.
In connection with the present invention, a set of curves is preferably obtained representing the measured crush strength of foam cement samples possessing one or more volume percent entrained gas as a function of the base cement compressive strength. A mathematical surface is generated with base cement compressive strength and percent entrained gas by volume as the X and Y coordinates respectively and foam cement compressive strength as the Z-axis. The foam cement compressive strength is obtained by measuring the base cement compressive strength by any method to determine the X axis value, and, knowing the volume percent entrained gas as the Y coordinate, determining the foam cement compressive strength from the Z-axis value of the mathematical surface.
The mathematical surface also may be expressed by an equation which is accessible by a control computer of a conventional compressive strength measurement system such as the UCA. If a UCA is used to measure the base cement compressive strength, the foam cement compressive strength may be computed over time as the cement sets, so long as the volume percent entrained gas is known. In the most preferred embodiment of the invention, the method thus comprises maintaining a base cement slurry sample at controlled temperature and pressure; transmitting an ultrasonic signal through the sample; detecting the ultrasonic signal subsequent to its transiting the sample and measuring the time required for said signal to transit the sample; determining, according to a predetermined relationship relating transit time to compressive strength, the compressive strength of the sample; determining, according to a predetermined relationship relating base cement compressive strength and volume percent entrained gas to foam cement compressive strength, the compressive strength of a foam cement having a given percent entrained gas by volume; and displaying or recording the compressive strength of said foam cement.
The present invention accordingly provides an efficient and cost effective method using established laboratory techniques to obtain foam cement compressive strength at temperature and pressure as a function of time. The method is convenient and efficient insofar as it utilizes easily obtainable laboratory data measured from multiple foam cement samples to predict the foam cement compressive strength on other samples of interest based only upon the base cement compressive strength and the percent entrained gas in the desired foam cement.
A better understanding of the present invention, its several aspects, and its advantages will become apparent to those skilled in the art from the following detailed description, taken in conjunction with the attached drawings, wherein there is shown and described the preferred embodiment of the invention, simply by way of illustration of the best mode contemplated for carrying out the invention.